1. Field of the Invention
The present invention relates to a position detection device for detecting an absolute position based on an image signal converted by a light-receiving element for receiving light irradiated from a light source toward a scale having a code pattern.
2. Description of the Related Art
In a positioning device such as a measurement device or a servomotor, accurate position detection is essential in order to realize high-level control. An encoder is an example of a device that is widely used to perform this kind of position detection. The encoder needs to be capable of highly accurate position detection as well as have a robustness having a high resistance to foreign matter and noise.
When high accuracy and a high level of robustness are required for position detection, an optical encoder is used. The optical encoder is usually configured from a light source, a scale having an equally spaced pattern or a pseudo-random code, and one or a plurality of light-receiving elements.
The optical encoder can be used for position detection by calculating a correlation between a signal waveform obtained from the light-receiving element and a known pattern marked on the scale.
Further, there has also been proposed a method (e.g., see Japanese Patent No. 5103267) that, by not only focusing on the signal waveform obtained from the light-receiving element but also on the edges detected based on a threshold, enables position detection at a resolution equal to or smaller than the code pattern by calculating a correlation between a binarized code pattern and a known pattern marked on the scale, and using a histogram of relative positions of the edges.
However, in the related art, there is a problem in that highly accurate position detection that relies on the relative positions of the edges is very susceptible to the affect of error detection of the edge positions. The error detection of the edge positions occurs due to signal disruptions caused by foreign matter that has worked itself onto the scale or the light-receiving element, or by electrical noise.